1. Field of the Invention
The present invention relates optical fibers and fabrication of optical fibers, and more particularly, the present invention relates to an optical fiber twisting apparatus that imparts a twist to the optical fiber during the fabrication process. The present invention also relates to a method of manufacturing an optical fiber using the optical fiber twisting apparatus, and to an optical fiber manufactured using this method of manufacturing.
2. Description of the Related Art
Conventionally, an optical fiber twisting apparatus is known whereby in order to reduce polarization mode dispersion (PMD) in an optical fiber there is provided a twisting roller. When an optical fiber is being manufactured by melting and drawing an optical fiber preform, the twisting roller imparts a twist to the molten portion of the optical fiber preform positioned on the upstream side of the optical fiber by oscillating while in contact with the optical fiber.
In this optical fiber twisting apparatus, melting and drawing of an optical fiber preform is performed while a twist is imparted to a suitable location on the optical fiber while the optical fiber preform is being wiredrawn. For example, a twist is imparted to a portion of the optical fiber preform that is melted in a drawing furnace by installing an optical fiber twisting apparatus and imparting a twist to the optical fiber using the twisting roller of the apparatus in a process performed after the optical fiber preform has been melted and wiredrawn, then cooled, then primarily coated and secondarily coated (see, for example, Japanese Patent No. 3224235, the disclosure of which is incorporated herein by reference in its entirety).
The twist that is imparted to the molten portion of the optical fiber preform by this optical fiber twisting apparatus is brought about by friction between the optical fiber twisting roller and the coated portion of the optical fiber. In order to efficiently impart a twist to the optical fiber, it is possible to extend the length of a contact portion between the surface of the optical fiber twisting roller and the optical fiber. In order to extend this contact length, it is possible to increase a contact angle of the optical fiber relative to the optical fiber twisting roller, or, if a large diameter optical fiber twisting roller is used, it is possible to further increase the winding force from the optical fiber twisting roller on the coated portion of the optical fiber. (see, for example, Japanese Patent No. 3224235).
However, if the contact angle is increased, or if a large diameter optical fiber twisting roller is used, then due to the effects of the accuracy, assembly tolerances, etc. of the optical fiber twisting roller, when the optical fiber twisting roller is being swung, the pressing force of the optical fiber twisting roller on the coated portion of the optical fiber changes which results in a non-uniform force being applied to the optical fiber during the drawing process. Consequently, problems arise such as irregular movement of the optical fiber rolling over the optical fiber twisting roller, and line distortion occurring such as the optical fiber moving to the left and right over the optical fiber twisting roller, so that consistent coating of the optical fiber with the primary coated portion and the secondary coated portion is no longer possible. As a result, there are large variations in the outer diameter of the optical fiber, and the thickness of the coated portion of the optical fiber varies in the longitudinal direction of the optical fiber.
Moreover, when the optical fiber twisting roller is being swung in order to impart a twist to the optical fiber, damage is caused to the coated portion of the optical fiber where the optical fiber twisting roller makes contact due to the effects noted above. Moreover, delamination occurs at a boundary between the optical fiber glass (i.e., the bare optical fiber) and the primary coated portion, and, if the optical fiber is put in a low temperature environment after it has been laid, the attenuation of the optical fiber is increased by this delamination of the coated portion.
The present invention was conceived in view of the above described problems and aspects of exemplary embodiments of the present invention provide an optical fiber twisting apparatus that prevents line vibration occurring in an optical fiber as it is undergoing a drawing process, making it possible to coat a bare optical fiber with consistency, and preventing damage to the coated portion of the optical fiber where the optical fiber twisting roller makes contact. Other aspects provide a method of manufacturing an optical fiber in which an optical fiber is manufactured using this optical fiber twisting apparatus.